Rechargeable prismatic batteries are used in a variety of industrial and commercial applications such as fork lifts, golf carts, uninterruptable power supplies, and electric vehicles.
Rechargeable lead-acid batteries are presently the most widely used type of battery. Lead-acid batteries are a useful power source for starter motors for internal combustion engines. However, their low energy density, about 30 Wh/kg, and their inability to reject heat adequately, makes them an impractical power source for an electric vehicles, hybrid electric vehicles and 2-3 wheel scooters/motorcycles. An electric vehicle using lead acid batteries has a short range before requiring recharge, require about 6 to 12 hours to recharge and contain toxic materials. In addition, electric vehicles using lead-acid batteries have sluggish acceleration, poor tolerance to deep discharge, and a battery lifetime of only about 20,000 miles.
Nickel metal hydride batteries ("Ni-MH batteries") are far superior to lead acid batteries, and Ni-MH batteries are the ideal battery available for electric vehicles and other forms of vehicular propulsion. For example, Ni-MH batteries, such as those described in copending U.S. patent application Ser. No. 07/934,976 to Ovshinsky and Fetcenko, the disclosure of which is incorporated herein by reference, have a much higher energy density than lead-acid batteries, can power an electric vehicle over 250 miles before requiring recharge, can be recharged in 15 minutes, and contain no toxic materials. Electric vehicles using Ni-MH batteries have demonstrated exceptional acceleration, and a battery lifetime of more than about 100,000 miles.
Extensive research has been conducted in the past into improving the electrochemical aspects of the power and charge capacity of Ni-MH batteries, which is discussed in detail in U.S. Pat. Nos. 5,096,667 and 5,104,617 and U.S. patent application Ser. Nos. 07/746,015 and 07/934,976. The contents of all these references are specifically incorporated by reference.
Ovonic Battery Company has developed high energy and high power nickel-metal hydride batteries with capacities ranging from 90 to 150 Ah for electric vehicle applications, and from 30-60 Ah for hybrid electric vehicle applications. Presently, for some applications, such as power assist HEV, even smaller capacities ranging from 5 to 20 Ah, are needed.
However, the aspect ratio of the electrodes of the battery becomes unfavorably wide when very short cans are employed. Therefore, for single cells of footprints suitable for EV size batteries (about 50 to 100 Ah) the practical lower limit on the capacity is around 25-30 Ah. To achieve high power HEV batteries of about 20 Ah, a smaller footprint can has been used, and it is projected that a smaller can would provide batteries of about 12-15 Ah. However, these smaller cells do not provide improvements in specific power or specific energy. In fact, the specific power and specific energy as well and power and energy densities decrease with lower size cans unless the electrode tab arrangements and cell hardware are redesigned and re-optimized.
The biggest problem with small batteries of the current prismatic battery design is that the cost per unit stored energy (specific cost) increases as the size decreases. This is because the cost of cell hardware components does not necessarily scale with size. Some costs are more related to the number of parts employed, which generally does not decrease at all with smaller batteries. For this reason, new battery designs are needed.
Additionally, it should be noted that until recently the mechanical and thermal aspects of the performance of Ni-MH batteries have been neglected. For example, in electric vehicles, the weight of the batteries is a significant factor because battery weight is the largest component of the weight of the vehicle. For this reason, reducing the weight of individual batteries is a significant consideration in designing batteries for electric powered vehicles. In addition to reducing the weight of the batteries, the weight of battery modules must be reduced, while still affording the necessary mechanical requirements of a module (i.e. ease of transport, ruggedness, etc.). Also, when these battery modules are incorporated into battery pack systems (such as for use in electric vehicles) the battery pack components must be as light weight as possible.
It should be particularly noted that electric vehicle applications introduce a critical requirement for thermal management. This is because individual cells are bundled together in close proximity and many cells are electrically and thermally connected together. Therefore, since there is an inherent tendency to generate significant heat during charge and discharge, a workable battery design for electric vehicles is judged by whether or not the generated heat is sufficiently controlled.
Sources of heat are primarily threefold. First, ambient heat due to the operation of the vehicle in hot climates. Second, resistive or I.sup.2 R heating on charge and discharge, where I represents the current flowing into or out of the battery and R is the resistance of the battery. Third, a tremendous amount of heat is generated during overcharge due to gas recombination.
While the above parameters are generally common to all electrical battery systems, they are particularly important to nickel-metal hydride battery systems. This is because Ni-MH has such a high specific energy and the charge and discharge currents are also high. For example, to charge a lead-acid battery in one hour, a current of 35 Amps may be used while recharge of a Ni-MH battery may utilize 100 Amps for the same one-hour recharge. Second, because Ni-MH has an exceptional energy density (i.e. the energy is stored very compactly) heat dissipation is more difficult than lead-acid batteries. This is because the surface-area to volume ratio is much smaller than lead-acid, which means that while the heat being generated is 2.5 -times greater for Ni-MH batteries than for lead acid, the heat dissipation surface is reduced.
The following illustrative example is useful in understanding the thermal management problems faced when designing Ni-MH battery packs for electric vehicles. In U.S. Pat. No. 5,378,555 to General Motors (herein incorporated by reference), an electric vehicle battery pack using lead acid batteries is described. The battery pack system, utilizing lead-acid batteries, has a capacity of about 13 kWh, weighs about 800 pounds, and has a vehicle range of about 90 miles. By replacing the lead-acid battery pack by an Ovonic battery pack of the same size, the capacity is increased to 35 kWh and vehicle range is extended to about 250 miles. One implication of this comparison is that in a 15 minute recharge, the power supplied to the Ni-MH battery pack is 2.7 times greater than that supplied to the lead-acid battery pack, with its commensurate added heat. However, the situation is somewhat different during discharge. To power a vehicle on the highway at constant speed, the current draw upon the battery is the same whether it is a Ni-MH battery or a lead-acid battery (or any other power source for that matter). Essentially the electric motor which drives the vehicle does not know or care where it gets the energy or what type of battery supplies the power. The difference between the heating of the Ni-MH battery and the lead-acid battery upon discharge is the length of discharge. That is, since the Ni-MH battery will drive the vehicle 2.7 times farther than the lead-acid, it has a much longer time before it has a chance to "cool-off".
Further, while the heat generated during charging and discharging Ni-MH batteries is normally not a problem in small consumer batteries or even in larger batteries when they are used singly for a limited period of time, large batteries that serve as a continual power source, particularly when more than one is used in series or in parallel, such as in a satellite or an electric vehicle, do generate sufficient heat on charging and discharging to affect the ultimate performance of the battery modules or battery pack systems.
Thus, there exists a need in the art for battery, battery module, and battery pack system designs which reduces the overall weight thereof and incorporates the necessary thermal management needed for successful operation in electric vehicles, without reducing its energy storage capacity or power output, increases the batteries' reliability, and decreases the cost.
Simply stated, the prior art does not teach an integrated battery configuration/internal design, battery module, and thermally managed battery pack system which is light weight, simple, inexpensive, and combines the structural support of the batteries, modules and packs with an air or water cooled thermal management system.